Its a common idiom to build a range by unioning other ranges into 
another one.  If this is done sequentially, those new ranges can be 
simply appended to the end of the existing range, avoiding some 
expensive processing fro the general case.

This patch identifies and optimizes this situation.  The result is a 
2.1% speedup in VRP and a 0.8% speedup in threading, with a overall 
compile time improvement of 0.14% across the GCC build.

Bootstrapped on  x86_64-pc-linux-gnu with no regressions. Pushed.

Andrew